Viscosity of a Ferrofluid

The viscosity of a ferrofluid is qualitatively similar in some respects to that of dilute to moderately concentrated suspensions discussed in Chapter 6. Useful ferrofluids often have 

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The preparation of a ferrofluid emulsions is quite similar to that described for double emulsions. The starting material is a ferrofluid oil made of small iron oxide grains (Fe203) of typical size equal to 10 nm, dispersed in oil in the presence of an oil-soluble surfactant. The preparation of ferrofluid oils was initially described in a US patent [169]. Once fabricated, the ferrofluid oil is emulsifled in a water phase containing a hydrophilic surfactant. The viscosity ratio between the dispersed and continuous phases is adjusted to lie in the range in which monodisperse fragmentation occurs (0.01-2). The emulsification leads to direct emulsions with a typical diameter around 200 nm and a very narrow size distribution, as can be observed in Fig. 1.33. 

Ferrimagnetic nanoparticles of magnetite (Fc304) in diamagnetic matrices have been studied. Nanoparticles have been obtained by alkaline precipitation of the mixture of Fe(II) and F(III) salts in a water medium [10]. Concentration of nanoparticles was 50 mg/ml (1 vol.%). The particles were stabilized by phosphate-citrate buffer (pH = 4.0) (method of electrostatic stabilization). Nanoparticle sizes have been determined by photon correlation spectrometry. Measurements were carried out at real time correlator (Photocor-SP). The viscosity of ferrofluids was 1.01 cP, and average diffusion coefficient of nanoparticles was 2.5 10 cm /s. The size distribution of nanoparticles was found to be log-normal with mean diameter of nanoparticles 17 nm and standard deviation 11 nm. 

Stepanov and Shliomis [80] have proposed an egg model akin to a three-dimensional form of the itinerant oscillator model [72]. (We recall [72] that in the itinerant oscillator, one relaxation mechanism occurs due to rotation of the oscillator as a whole, the other due to jumping of the inner rotator over a potential barrier) whereby the interdependence of the two relaxation mechanisms is taken into account. In the egg model [80], the ferrofluid particle is represented as an egg of volume V (the hydrodynamic volume) embedded in the liquid with viscosity 17. The magnetic moment of the ferroparticle is modeled by the yolk of volume V magnetic torque acts directly on the yolk but due to the viscosity of the white it is also transmitted to the eggshell. Such a model allows one to take into account the coupling between the two mechanisms in a quantitative way so that its influence on and may be estimated. 

Offset constant of the applied magnetic field Ferrofluid/solid interface inside V Gravitational amplification factor of a-phase Goeffrcient in the Muller porosity model Porous medium porosity Bed holdup of a-phase Vortex viscosity Dynamic viscosity... 

The carrier liquid of the ferrofluid used in the experiment (APG S lOn, Ferrotec) was a synthetic ester oil which was immiscible in silicone oil. The viscosity rj, density /O, and surface tension a of the ferrofluid at 25 °C were 0.406 kg/ms, 1330 kg/s, and 32xlO N/m, respectively. The saturation magnetization and the initial susceptibility X of this ferrofluid were 44 mT and 1.6, respectively. Two silicone oils with different viscosities were used to investigate the effect of viscous friction [—Si(CH3)20—] , with a kinematic viscosity of V = 50 cSt, and [—C7H80Si—], with a kinematic viscosity of 100 cSt. Both oils had approximately the same surface tension of 2.03 x 10 N/m and a density of 960kg/m. In the experiments, the ferrofluid droplet was surrounded... 

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